Ideally, digital images created through the use of CMOS and other solid state image sensors are exact duplications of the imaged scene projected upon the image sensor arrays. However, pixel saturation, analog-to-digital saturation, exposure, gain settings and bit width processing limitations in an imaging device can limit the dynamic range of a digital image.
To achieve a more realistic photographic image, an automatic camera must choose an appropriate exposure time and gain setting for image pixel signals using available dynamic range information. Generally, using higher gains and longer exposures result in the brightest areas of a scene being clipped. On the other hand, using lower gains and shorter exposures result in darker, noisier pictures. Therefore, to choose an optimal exposure while preventing clipping of the brightest areas, the camera's auto-exposure module relies on having scene luminance distribution and maximum scene luminance information available, which are affected by limitations on the dynamic range of the camera.
FIG. 1 is an image which shows a dynamic limitation based on pixel saturation. It includes a person in the foreground and a background containing a bright lamp. Both the person and the bright lamp are imaged without loss of detail, however the person looks underexposed. Merely increasing the exposure period may more correctly expose the person but would lead to brightness clipping and loss of detail of the lamp, as shown in FIG. 2. Clipping also occurs when a pixel output signal is limited in some way by the full range of an amplifier, analog-to-digital converter or other circuit within an imaging device that captures or processes the pixel signals of an image. When clipping occurs, pixel signals are flattened at the peak luminance values. As another example of pixel signal clipping due to circuit limitations, a sensor equipped with a 10-bit ADC would typically have an image processing pipeline with pixel values in the range 0 to 1023 Least Significant Bits (LSB). Thus, luminance that corresponds to a pixel value above 1023 LSB is clipped to 1023 LSB, resulting in a loss of information. Accordingly, there is a highest scene luminance DScene HI and a highest luminance DCam HI which can be sensed and processed by the camera without clipping.
Particularly, in scenes where an exposure setting may properly expose most of the scene while clipping some areas of the scene, as shown in FIG. 2, there is a need for the auto-exposure operation to have scene luminance information that exceeds DCam HI. If a decrease in exposure setting largely prevents the brightest areas of a scene from clipping without substantially underexposing the main subject, the auto-exposure operation should proceed further using the reduced exposure setting. However, if a decrease in exposure setting is insufficient to prevent the brightest areas of a scene from clipping without substantially underexposing the main subject, the auto-exposure operation should keep the exposure setting unchanged.
Additionally, when scene illumination changes rapidly, the rapid increase in scene luminance may exceed DCam HI and saturate the image output, thereby resulting in a loss of information on scene brightness to the auto-exposure module. For example, a person may use her camera phone, place it on a table, later pick it up and attempt to take a picture immediately thereafter. While the camera phone is lying on the table, the camera lens faces the table surface and the luminance of the table is very low so that the auto-exposure module sets a long exposure time. As the person picks the phone up and points the camera at a scene to be captured, e.g., a bright outdoor scene viewed through a window, the scene luminance changes by many orders of magnitude within seconds. The excessive illumination causes the image to saturate and become clipped. Consequently, the auto-exposure module has insufficient information on the actual brightness of the scene and cannot accurately determine the appropriate exposure for the image. There is therefore a need to capture as much luminance information as possible from a pre-capture image for use in auto-exposure operations so these operations may be more quickly performed to set a proper exposure for image capture.